Projectiles to trigger avalanches

ABSTRACT

A projectile for being propelled along a trajectory by propellant gases inside a launcher tube, the projectile including a casing, first pressure means for first pressurizing the casing without exploding the casing during firing of the projectile, and second pressure means for second pressurizing and exploding the casing, wherein the first pressure means includes part of propellant gases inside a launcher tube used to propel the projectile, or alternatively, gases generated by the first pressure means.

FIELD OF THE INVENTION

The technical scope of the invention is that of devices, and notablyprojectiles, able to trigger an avalanche.

It is known to artificially trigger avalanches, this in order to preventtoo great an accumulation of snow in an avalanche corridor therebyreducing the risks to the buildings and persons located on the lowerpart of the slope.

BACKGROUND OF THE INVENTION

Known devices enabling avalanches to be triggered are either fixed ormobile.

Fixed installations are costly. Indeed, the infrastructure are generallybuilt in places that are difficult to reach, and they also requireelectrical or fluid (combustible gas) connections, which are difficultto ensure.

Thus, patent FR-2771168 describes an avalanching device that pumps upballoons using an explosive gas.

Patent FR-2636729 proposes the permanent installation of an explosivegas generating ramp oriented towards the slope.

Other devices more often than not implement a compressed air gun thatlaunches an explosive projectile triggered by an impact fuse. U.S. Pat.No. 5,872,326 describes such a projectile.

These devices also have drawbacks.

Thus, the conditions of use are limited by the safety of the explosivesbeing implemented (transport, storage, loading). Moreover, in the eventof a malfunction, an explosive projectile risks being present on theground, thereby posing a risk to safety and the environment.

Up to now, these risks have been attenuated through the use of dualcomponent explosives. Two components are mixed in the projectile bodyin-situ before firing. Individually the components are inoffensivethereby ensuring their safe transportation and storage. The mixture isexplosive but becomes inert after a period of 48 hours therebyeliminating risks linked to the abandonment in situ of non-ignitedprojectiles.

However, the projectiles are difficult to implement since they requirethe components to be mixed together in situ. This operation is madedifficult by the climatic conditions (cold, damp) and the topography ofthe terrain (hilly). Thus, in practical terms, known projectiles areeither fired from a fixed platform, or brought vertically by helicopterin the vicinity of the avalanche corridor. Once again, implementation isboth complicated and costly.

A further drawback lies in the event of the inadvertent suspension offire for whatever reason (bad weather, launcher breakdown . . . ). Ifthe explosive mixture has been made and firing is not possible, then thelive projectile has to be kept in storage for 48 hours.

SUMMARY OF THE INVENTION

The aim of the invention is to propose a projectile to triggeravalanches that overcomes such drawbacks.

Thus, the projectile according to the invention is simple to implementsince it does not require an explosive mixture to be prepared in situ.It may be easily implemented whatever the terrain and notably usinglight launchers that can be brought by trackers.

It offers an excellent level of safety, both before and after firing, inthe event of a triggering failure.

Thus, the invention relates to a projectile to trigger avalanches andintended to be projected by a launcher tube, such projectile comprisinga casing intended to explode in the vicinity of or in contact with thesnow through the action of priming means in order to cause an avalanche,wherein said casing is able to be pressurized during firing and/orduring its trajectory, the pressurization of said casing thus obtainedbeing insufficient by itself to ensure the exploding of the casing,other means being provided to overpressure the casing thereby ensuringits exploding.

The pressurisation of the casing during firing may be obtained by usingpart of the propellant gases, inside the launcher tube, used to fire theprojectile.

The casing may be pressurized during firing and/or during trajectory bymeans of first gas-generating means integral with the projectile andignited during firing.

According to a first embodiment, the projectile comprises a pistonpushed by the gas pressure supplied by the launcher or by the first gasgenerator and allowing the gases to enter the casing, such piston beingbrought back into a closing position by a return spring and ensuring thegas pressure is maintained inside the casing.

This projectile will also comprise a second gas generator activated bypriming means enabling an overpressure of the casing to be ensuredcausing it to fracture.

Advantageously, the gas pressure generated by the second generator willnot be enough to ensure the fracturing of the casing on its own.

The second gas generator may comprise a pyrotechnic composition or apowder charge ignited by priming means.

The priming means may comprise a percussive fuse to ignite the secondgas generator when the projectile impacts on the ground.

The projectile may comprise a combustion monitoring device for thepyrotechnic composition or the powder charge.

According to a second embodiment, the second gas generator may ensurethe generation of a combustible and/or explosive gas that will beignited upon impact by priming means.

The second gas generator may comprise calcium carbide that will be mixedwith water during the trajectory, the water being contained in areservoir that will be opened by opening means activated during firing.

The opening means may comprise a riser head able to translate againstthe action of a return spring, such riser head being pushed towards thereservoir through the inertial force deployed during firing, therebyensuring the fracturing of the reservoir.

The priming means may comprise a percussive primer placed at a frontpart of the projectile and ignited by a percussion device.

According to a third embodiment, the first gas generator may ensure thegeneration of a combustible and/or explosive gas.

The first gas generator may comprise calcium carbide that is mixed withwater during the trajectory, the water being contained in a reservoirthat will be opened by opening means activated during firing.

The opening means may comprise a piston sliding in the reservoir againstthe action of a return spring, the piston being displaced during firingthrough the action of the propellant gases and carrying a pin allowingthe reservoir to be pierced thereby ensuring that the water and calciumcarbide come into contact with one another.

The calcium carbide may be placed in a spray tube perforated with radialholes, such tube being coaxial to the projectile and placed in theprolongation of the reservoir.

According to one variant, the return spring may be made of ashape-memory material selected and parametered such that it retractswhen it reaches a temperature beneath a certain rate and thus no longerexerts the same return force on the piston.

The priming means may comprise a percussive primer placed at a frontpart of the projectile and ignited by a percussion device.

The priming means may comprise at least one detonating cord placed on aninternal surface of the casing.

In any event, the priming means may comprise a primer connected to delaymeans ignited during firing.

The projectile may comprise a controlled leak device ensuring thegradual depressurisation of the casing.

The controlled leak device may comprise at least one interior cap madeof a porous material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more apparent after reading the description ofthe different embodiments, such description being made with reference tothe appended drawings, in which:

FIG. 1 shows a simplified longitudinal section of a projectile accordingto a first embodiment of the invention,

FIG. 2 shows a simplified longitudinal section of a projectile accordingto a second embodiment of the invention,

FIG. 3 shows a simplified longitudinal section of a projectile accordingto a third embodiment of the invention,

FIG. 4 shows a variant of the piston used in the third embodiment,

FIG. 5 shows a variant embodiment of a depressurisation cap,

FIG. 6 shows a front part of a projectile according to a thirdembodiment and equipped with a variant of the priming means,

FIG. 7 also shows a front part of a projectile according to the thirdembodiment equipped with another variant of the priming means,

FIGS. 8a and 8 b shows particulars of variant embodiment of theprojectile casing.

PREFERRED EMBODIMENTS

With reference to FIG. 1, a projectile 1 according to a first embodimentof the invention, comprises a casing 2 having a cone-shaped front part 2a and a rear part 2 b, constituting an aerodynamic stabiliser, and whichis formed of a cone 3 followed by a cylindrical part 4.

The casing 2 may be made of a metallic material, for example 3 mm thickaluminium, or else a composite material, for example carbon fibre orKevlar filament winding.

The casing 2 accommodates an inner tube 5 that presses by a front seat 5a on an inner surface of the casing 2 and which has an enlarged rearpart 5 b fitted to a cylindrical bore 6 in the casing 2. The tube 5 willbe made, for example, of a plastic material or of an aluminium alloy.

A stop ring 7 ensures the axial immobilisation of the tube 5 withrespect to the casing 2.

The tube 5 has two chambers 8 a, 8 b separated by a wall 9. The rearchamber 8 a encloses a sliding piston 10 that is pushed by a returnspring 11 and abuts against a stop nut 12 screwed inside the tube 5.

The piston 10 has a rear sealing lip 10 a.

The axial part of the piston 10 receives a cap 22 made of a porousmaterial, for example sintered bronze.

This cap 22 constitutes a controlled leak device enabling a slow andgradual depressurisation of the inside of the casing 2.

Radial holes 13 are arranged in the tube 5 between the wall 9 and thepiston 10 when the latter is in its starting position shown in FIG. 1(in abutment against the nut 12).

By way of a variant, the rear chamber 8 a can also receive a first gasgenerator 19 that will comprise a primer 20 ignited by the gasessupplied by the launcher system (not shown) itself igniting a gasgenerating pyrotechnic composition of a known type.

The front chamber 8 b encloses a second gas generator 18 as well as itspriming system 14.

This comprises a percussive fuse that is not shown here in details andwhich incorporates in a known manner a safety and arming device (SAD)15, a firing pin 16 activated by inertia during impact upon the groundand a percussive primer 17.

The SAD 15 ensures that the firing pin 16 is locked in place duringstorage phases. It releases the firing pin when the projectile is firedand thus incorporates an inertial lock (not shown). SADs are well knownto someone skilled in the art and it is therefore unnecessary todescribe such a SAD in further detail.

The primer 17 is intended to ignite the second gas generator 18 that isformed, for example, by a gas generating pyrotechnic composition. Gasgenerating compositions are well known to the expert. Reference may bemade, for example, to U.S. Pat. No. 5,062,367, FR-2691706 and EP-0509655which describe gas generators that may be used in automobile safetydevices.

The second gas generator may also be constituted by a propellant powdercharge.

A device 21 to monitor the ignition of the second gas generator 18 isarranged at a front part 2 a of the casing 2.

This monitoring device 21 will be constituted, for example, by analuminium rivet carrying, on the external face of the projectile, anaxial hole (not shown), that is blocked off, and inside which a fusiblematerial is placed, for example a plastic material (such as polystyrene)or else an eutectic alloy.

The rear face of the rivet 21 is in contact with the composition 18 orwith a case enclosing it. The heat given off by its combustion ensuresthe liquefaction of the fusible material placed in the rivet 21.

This projectile operates as follows.

According to a preferred variant of the invention, no first gasgenerator 19 is provided.

The projectile 1 is installed into a launcher tube (not shown), forexample a pneumatic launcher tube supplying pressurized gas, or else alight launcher for pyrotechnic charges (for example, of the typedescribed in patent FR-2576682).

The gas pressure supplied by the launcher is applied to the piston 10which is pushed against the action of the return spring 11. The piston10 thus releases the holes 13 which allow a passage for the gasessupplied by the launcher. The gas is thus pressurized inside the casing2.

When the projectile exits the launcher tube, the pressure applied to therear of the piston 10 is reduced. The pressure of the gases enclosed inthe casing push (with the spring 11) the piston 10 against its stop nut12. The casing is thereby pressurized during firing.

By way of example, for a pneumatic launcher having a range of 3000 m, agas pressure of 400 bars is supplied by the launcher which is enough topressurise the inside of the casing to a pressure of 200 to 300 bars.

The casing 2 will be defined so as to be able to withstand the initialpressurisation without damage.

If the launcher is not able to supply enough gas pressure (which wouldbe the case, for example, for a compact short-range launcher) first gasgenerating means 19 will be provided. This generator will be ignitedwhen the projectile is fired, for example through the action of the hotgases supplied by the launcher and applied directly on to the primer 20.An inertia operated firing pin system may also be provided that willcause the ignition of this generator 19 during firing.

The first gas generator 19 will ensure the casing 2 is pressurizedaccording to the mechanism described above: displacement of the piston10, entrance of the gases in the casing 2 through the holes 13, returnof the piston 10 when the internal pressure of the casing (added to theforce supplied by the spring 11) exceeds that exerted to the rear of thepiston 10.

During firing the inertial forces caused the SAD 15 to unlock, allowingthe firing pin 16 to ignite the primer 17.

Upon impact on the ground, the deceleration to which the projectile issubjected causes the primer 17 to ignite and prime the second gasgenerator 18.

This is dimensioned so as to ensure an overpressure of the casing 2causing it to fracture. The avalanche results from the exploding of thecasing 2.

By way of example, a casing 2 may be provided made of 3 mm thickaluminium. This casing can withstand without damage a pressure of 300bars. It will explode at a pressure equal to or in excess of 400 bars.

The initial pressurisation supplied by the launcher or the first gasgenerator will ensure a pressurisation inside the casing of around 200to 300 bars.

The second gas generator will be dimensioned, for example, so as tosupply a pressure of around 100 bars.

Thus, the casing will explode as a result of the gas generator beingtriggered upon impact on the ground.

Advantageously, the gas pressure generated by the second gas generatorwill be selected under that required to fracture the casing alone.

Thus, in the event of the second gas generator being accidentally primedduring the transportation or storage phases, the pressure generated willnot be sufficient to fracture the casing.

If a monitoring device 21 is provided on the projectile, theaccidentally priming of the gas generator 18 will be revealed by thefusion of the material placed in the ignition monitoring device.

The pressure generated by this second gas generator will graduallyevacuate via the porous cap 22. The porous cap 22 carried by the piston10 constitutes a controlled leak enabling the gradual depressurisationof the casing 2.

Thus, if an incident should occur related to the second gas generator 18and the projectile 1 does not explode, the pressure inside the casingwill gradually reduce. The porosity will be selected so as to ensuredepressurization in approximately 48 hours. Non-exploded projectilesfound on the ground after the snow has melted therefore present nodanger since they will not contain any pressurized gas.

Moreover, the accidental pressurising of the gas generator 18 when theprojectile is being picked up will not be dangerous because this wouldnot be enough to cause the casing 2 to fracture.

FIG. 2 shows a projectile according to a second embodiment of theinvention.

This projectile differs from the first one only in that the structure ofthe second gas generator is different.

Here, the second gas generator is designed so as to ensure thegeneration of a combustible and/or explosive gas that may be ignitedupon impact by priming means.

The second gas generating means comprise a housing 23 filled withcalcium carbide 24 in the form of granules.

This calcium carbide is intended to be mixed with water during thetrajectory. The water is contained in a reservoir 25 made of a plasticmaterial or of glass and opened by opening means 26 activated by inertiaduring firing.

A ring-shaped wall 27 is placed at the median part of the front chamber8 b and separates the calcium carbide 24 from the water reservoir 25 andits opening means 26.

The ring-shaped wall is made integral with the tube 5 by two flexiblerings 28 a, 28 b.

According to the embodiment presented, the opening means 26 comprise ariser head 29 able to translate against the action of a return spring 30fixed to the ring-shaped wall 27.

This riser head is pushed towards the reservoir 25 by the inertial forceduring firing, thereby fracturing the reservoir.

These opening means are presented here merely by way of illustration.Other inertial opening means may naturally be envisaged. For example, areservoir 25 may be provided that is itself able to translate throughinertia during firing and which impacts against a point integral withthe wall 9 of the tube 5.

In a known manner, the mixture of water and calcium carbide causes thegeneration of acetylene. This gas fills the tube 5. The relative massesof calcium carbide and water will be selected by someone skilled in theart so as to generate the required acetylene gas pressure.

The quantity of gas will be selected to be insufficient upon ignition tofracture the casing 2 of the unpressurized projectile.

The priming means here comprise a percussive primer 31 that is placed atthe front part 2 a of the projectile and which is ignited upon impactingthe ground by a percussion device (see FIG. 7), for example a firing pindisplaced by the impact on the ground.

This embodiment operates in a similar manner to the previous one.

The projectile casing is unpressurized during the storage and transportphases. Thus, the projectile is totally reliable and safe. Even theaccidental ignition of the second gas generator, if it explodes the tube5, is not enough to cause the projectile casing 2 to fracture.

Upon firing the casing 2 is pressurized, either by using the gasesproduced by the launcher, which penetrate into the casing via the holes13 after the piston 10 is displaced, or else by using the gasesgenerated by a first gas generator 19.

At the same time, the water mixes with the calcium carbide and the frontchamber 8 b of the tube 5 is filled with an explosive gas.

Upon impacting on the ground, the acetylene is ignited by the primer 31.This results in the tube 5 exploding and an overpressure that causes theprojectile casing 2 to fracture.

As in the previous embodiment, the non-ignition upon impact on theground has no impact on safety. Indeed, the pressure inside theprojectile will gradually decrease thanks to the controlled leakage ofthe gases through the porous cap 22.

FIG. 3 shows a projectile 1 according to a third embodiment of theinvention.

This embodiment differs from the previous ones notably in that it onlyintegrates a first gas generator 32 ensuring the generation of acombustible and/or explosive gas that fills the whole of the projectilecasing 2.

This first gas generator comprises a tubular sprayer 33 that extendssubstantially over the full length of the casing 2 between a reservoir34 and the cone-shaped part of the casing 2.

The sprayer 33 presses by a front seat 33 a on an inner surface of thecasing 2 and is positioned in a centering collar 35 of the reservoir 34.

The reservoir 34 is held in place axially with respect to the casing bymeans of a flexible ring 7. It is globally cylindrical in shape and isfitted in the bore 6 in the casing.

The spray 33 is perforated in its front part by radial holes 36 and itcontains granulated calcium carbide 37. A cylindrical metallic mesh maybe placed in the sprayer so as to keep the granules in place and preventthem from exiting through the holes 36.

The reservoir 34 is made of a plastic material. It contains water 38 aswell as opening means 39.

The opening means 39 comprise a piston 40 mounted sliding in thereservoir 34 against the action of a return spring 41.

The piston 40 carries a pin 42 that is able to perforate the reservoir34.

The reservoir 34 is closed by a ring-shaped nut 43 and a seal 44 isplaced between the piston 40 and the nut 43.

This projectile operates as follows.

During firing, the gas pressure from the launcher is exerted on thepiston 40 which is pushed towards the front of the projectile.

The reservoir 34 is not completely filled with water (the water level 38has been indicated in the Figure), the displacement of the piston beingthus made possible until the reservoir 34 is perforated by the pin 42.

Upon exiting the tube, the pressure exerted on the piston 40 is reducedand the spring 41 brings it back to press against the nut 43.

The water 38 is thus brought into contact with the calcium carbide 37and the acetylene thereby generated fills the projectile casing 2 viathe holes 36 of the sprayer 33.

As in the previous embodiment, the priming means comprise a percussiveprimer 31 that is placed at the front part 2 a of the projectile and isignited upon impact on the ground by a percussion device (not shown),for example, a firing pin displaced by the impact on the ground.

The acetylene is detonated and the resulting overpressure explodes thecasing 2.

By way of a variant, a mass of calcium carbide and water may be providedsuch that the quantity of acetylene generated is enough for the casingto be exploded merely by the impact of the projectile on the ground.

In this case, there is no need to provide a percussive primer 31.

In the event of a misfire, it is crucial to prevent a pressuriseprojectile containing an explosive gas from being left on the ground.

Means will therefore be provided to ensure the emptying of theprojectile casing.

The return spring 41 may, for example, be made of a shape-memorymaterial.

This material will be selected such that it retracts when it reaches atemperature beneath a certain rate and thus no longer exerts the samereturn force on the piston.

Winter temperatures will cause the spring to be returned to the startingposition in which it no longer applies the piston 40 against the nut 43.Advantageously, one end of the spring will be integral with the piston.The retraction of the spring will therefore drive the piston.

Sealing is thus no longer ensured and the gas is able to escapegradually from the casing 2 via the hole 43.

By way of a variant, other means may be provided to ensure the emptyingof the projectile casing 2.

FIG. 4 thus shows a detail of a variant in which the piston 40incorporates a porous ring-shaped part 45 made, for example, of sinteredmetal and whose porosity is selected such that the water is held in thereservoir but the gases are able to evacuate gradually.

FIG. 5 shows another variant in which a cap 46 of porous material isplaced directly onto the casing 2. This variant may also be associatedwith the embodiments shown in FIGS. 1 and 2.

FIG. 7 shows a variant of a third embodiment of the invention in which,so as to facilitate the fracturing of the projectile casing 2, adetonating shearing cord 49 has been provided fastened to the innersurface of the casing 2, for example by bonding (detonating cords arewell known to the expert).

Such a variant also improves the ignition of the explosive gas fillingthe casing 2.

FIG. 7 also shows a percussive primer 31 that ignites the detonatingshearing cords 49 as well as the mechanical firing pin 50 associatedwith it. This firing pin is retained with respect to a case 51 by ashearable collar that is fractured during impact.

Other priming means may also be used with one or other of the previousembodiments.

Thus, FIG. 6 shows a front part of a projectile that carries primingmeans comprising a primer 48 activated by delay means 47 ignited duringfiring. A programmable timer delay (for example electronic) may beprovided or else a pyrotechnic delay comprising a delay composition thatwill be ignited during firing.

Such delay means are not described in further detail and are well knownto the expert.

Such a variant allows the projectile to be primed whatever the nature ofthe ground, notably the hardness of the snow. The delay time beforeignition will be programmed before firing depending on the range atwhich the projectile is launched. Ignition will be programmed a littlebefore impact with the snow, or else after the projectile has becomeburied in the top layer of snow, according to conditions.

Depending on the case, the primer 48 will either ignite the acetylene(embodiment in FIGS. 2 and 3) or the gas generator 18 (embodiment inFIG. 1).

In all the embodiments previously described the pressure causing thecasing 2 to fracture may be accurately calibrated by providing incipientfractures on the casing, for example thinned areas.

FIG. 8a thus shows the longitudinal incipient fractures 52 that areevenly spaced angularly and which extend over substantially all thelength of the casing.

FIG. 8b shows ring-shaped incipient fractures 53 that are evenly spacedaxially with respect to the casing 2.

What is claimed is:
 1. A projectile for being propelled by propellantgases inside a launcher tube and being launched therefrom into atrajectory, the projectile comprising: a casing; first pressure meansfor permitting first pressurizing the casing without exploding thecasing during propelling of the projectile; and second pressure meansfor second pressurizing and exploding the casing, wherein said firstpressure means comprises means for receiving propellant gases within alauncher tube.
 2. A projectile for being propelled by propellant gasesinside a launcher tube and being launched therefrom into a trajectory,the projectile comprising: a casing; first pressure means for permittingfirst pressurizing the casing without exploding the casing duringpropelling of the projectile; and second pressure means for secondpressurizing and exploding the casing, wherein said first pressure meanscomprises a first gas-generating means integral with said projectile forgenerating gas, said first gas-generating means for ignition bypropellant gases inside a launcher tube.
 3. The projectile according toclaim 1, further comprising a first return spring and a piston operativeby gas pressure inside a launcher tube and for allowing gases in alauncher tube to enter and pressurize said casing, wherein the firstreturn spring is for urging said piston to a closed position to seal thepressurized casing.
 4. The projectile according to claim 3, wherein saidsecond pressure means comprises a second gas generator activated by apriming means, said second gas generator for pressurizing said casing tocause said casing to fracture.
 5. The projectile according to claim 4,wherein casing is not fracturable by gas pressure generated by saidsecond generator.
 6. The projectile according to claim 4, wherein thesecond gas generator comprises a gas generating composition, said gasgenerating composition comprising a pyrotechnic composition for ignitionby the priming means.
 7. The projectile according to claim 4, whereinsaid priming means comprises a percussive fuse for igniting said secondgas generator when said projectile impacts the ground.
 8. The projectileaccording to claim 6, further comprising a combustion monitoring devicefor monitoring the gas generating composition.
 9. The projectileaccording to claim 4, wherein the second gas generator ensuresgeneration of a gas selected from the group consisting of an explosivegas that is ignited upon impact by said priming means.
 10. Theprojectile according to claim 9, further comprising a reservoircontaining water and an opening means, wherein said second gas generatorcomprises calcium carbide for being mixed with water during thetrajectory, said opening means for opening the reservoir duringpropelling of the projectile.
 11. The projectile according to claim 10,further comprising a second return spring, wherein the opening meanscomprises a riser head for translating against an urging force of thesecond return spring, said riser head for being pushed towards thereservoir by inertial force deployed during firing, thereby ensuringfracturing of said reservoir.
 12. The projectile according to claim 9,further comprising a percussion device, wherein said priming meanscomprises a percussive primer located at a front part of the projectileand for ignition by the percussion device.
 13. The projectile accordingto claim 2, wherein said first gas generator generates an explosive gas.14. The projectile according to claim 13, further comprising a reservoirand opening means containing water, wherein said first gas generatorcomprises calcium carbide for mixing with the water after the projectilehas been launched into a trajectory, said opening means for opening thereservoir during propelling of the projectile.
 15. The projectileaccording to claim 14, wherein said opening means comprises a piston forsliding in the reservoir and operable by inertial force duringpropelling of the projectile and by a predetermined return forcegenerated by a return spring, and a pin carried by the piston forpiercing the reservoir to cause the water and the calcium carbide tocontact each other.
 16. The projectile according to claim 15, furthercomprising a spray tube perforated with radial holes and containingcalcium carbide, said tube being coaxial with the projectile andextending from the reservoir.
 17. The projectile according to claim 15,wherein said return spring is made of a shape-memory material forretracting when the material is at a first temperature no greater than apredetermined temperature, wherein at said first temperature said returnspring no longer exerts the predetermined return force on the piston.18. The projectile according to claim 13, further comprising apercussion device, wherein the priming means comprises a percussiveprimer located at a front part of the projectile and for ignition by thepercussion device.
 19. The projectile according to claim 13, whereinsaid priming means comprises at least one detonating cord located on aninternal surface of said casing.
 20. The projectile according to claim1, further comprising a delay means, wherein said priming meanscomprises a primer connected to the delay means for ignition duringpropelling of the projectile.
 21. The projectile according to claim 1,further comprising a controlled leak device for gradual depressurizationof the casing.
 22. The projectile according to claim 21, wherein saidcontrolled leak device comprises at least one interior cap integral withthe casing and comprising a porous material.
 23. The projectileaccording to claim 1, further comprising a first return spring and apiston operative by gas pressure supplied by said first gas generatorand for allowing gases supplied by said first gas generator to enter andpressurize said casing, wherein the first return spring is for urgingsaid piston to a closed position to seal the pressurized casing.
 24. Theprojectile according to claim 6, wherein said gas generating compositionis a powder charge ignitable by said priming means.
 25. The projectileaccording to claim 21, wherein said controlled leak device comprises aporous material integral with the piston.
 26. The projectile accordingto claim 21, wherein said first gas-generating means comprises aninertia operable firing pin for ignition of said first gas-generatingmeans.